PROJECT SUMMARY Blood liquid biopsy is an advanced emerging method in clinical oncology that refers to detecting circulating tumor markers in the blood samples. Among various circulating markers (e.g., tumor cells, cell aggregates, tumor-associated exosomes and DNA), circulating tumor cells (CTCs) have great advantage because they are direct `messengers' of deadly cancer metastasis. However, despite efforts, blood liquid biopsy for testing blood CTCs have had limited success and many patients still die of metastasis. One of the critical barrier is that CTCs can escape from a blood by taking lymphatic route for dissemination that is widely believed playing a key role in early metastasis. However lymph liquid biopsy for lymphatic CTCs (L-CTCs) analysis has not been developed due to technical difficulties to find and cannulate transparent, low pressure and tiny lymphatic vessels and collect a lymph sample. As a solution, we propose to use patient-derived orthotopic xenograft (PDOX) animal models, in which a patient's primary tumor tissue implanted into immunocompromised mice provides recapitulation of disease progression of the host-patient. The goal of this proof-of-concept clinical research is to develop lymph liquid biopsy for testing L-CTCs in triple-negative (TN) breast cancer. Our central concept (hypothesis) is that the quantitative, molecular and genetic analysis of L-CTCs using lymph liquid biopsy will reveal novel advanced diagnostic and prognostic markers of early metastasis. To increase accuracy, we suggest to integrate L-CTC analysis with well-established blood biopsy and with highly sensitive in vivo counting of L-CTCs using our technique of lymph flow cytometry. Breast cancer was selected as the disease model because its early diagnosis remains a serious healthcare problem; and it exhibits early lymphatic dissemination. Among other types, TN breast cancer is associated with poor clinical outcome. We will accomplish our goal through two specific aims: (1) Develop lymph liquid biopsy using primary tumor tissue from patients with early stage of TN breast cancer and PDOX mouse model; (2) Determine the clinically relevant significance of L-CTCs. If successful, lymph fluid biopsy using PDOX models holds great promise to obtain information on L-CTC genes and molecules driven metastasis that would be used to novel personalized screen for disease prognosis in patients with TN breast cancer. As a long-term goal lymph liquid biopsy offers an opportunity for to develop and test clinically relevant personalized treatments for individual patients.